1. Field of the Invention
The invention relates to a process for cleaning or maintaining the cleanliness of a low-consistency branch of a stock density-regulated headbox system including the constant part of a paper or cardboard machine with at least one high-consistency branch and a low-consistency branch, where the final concentration output by the headbox onto a wire or between two wires is determined by the mixing ratio of high-concentration and low-concentration suspension flows.
The invention further relates to a headbox system including the constant part of a paper or cardboard machine with a stock density regulation that is sectioned across the machine width by variably mixing a high-consistency suspension and a low-consistency suspension, having a high-consistency branch and a low-consistency branch.
2. Discussion of Background Information
Sectional stock density-regulated headbox systems are generally known from countless patent applications and disclosures. For example, reference is made to Applicant's German Patent Disclosure Document DE 40 19 593 A1, which discloses a sectional stock density-regulated headbox with two branches of individual supplies that are distributed across the machine width and are for two stock suspensions that have different concentrations. In this instance, paper stock and backwater are used as the suspensions with the different consistencies. With variances in the basis weight cross section of the paper web at a particular point of the web width, which is associated with a particular section of the headbox, the concentration C.sub.M of the relevant section flow is corrected by changing the quantity ratio of regulating flows that are supplied to a mixer and have different solids contents Q.sub.H /Q.sub.L. This type of basis weight cross section regulation is very advantageous with regard to a simultaneously good fiber orientation cross section and causes very favorable results with regard to the basis weight cross section.
The disclosure of this previously cited reference and Applicant's earlier Patent Applications DE 37 41 603 A1, DE 44 22 907 A1, DE 42 37 304 A1, and DE 42 11 291 A1, particularly with regard to the possibilities for density regulation of the final concentration of stock suspensions by mixing suspensions that have different concentrations, are herein incorporated by reference in their entireties. In particular with regard to the cited prior art, it should be emphasized that possible basis weight changes that are produced by metering particular fibrous materials or fibrous material contents can be compensated for in the known headboxes by means of a corresponding regulation mechanism that influences the sectional concentration of the stock suspension. The cited prior art discloses in detail how the supply to the headbox of stock suspensions that have different properties can be carried out and how the basis weight cross section of the material web can be kept uniform.
In practice, with the above-cited stock density-regulated headbox systems, the desired final concentration is usually regulated by means of a more or less intensive admixing of backwater I or clear filtrate to a slightly over-concentrated stock suspension, where backwater I is understood to mean all of the circulating backwater. In this connection, it has turned out that increased scale and slime deposits can occur in the supply lines of the low concentrate. As a result of this, on the one hand, the regulation mechanisms can be impaired, because the hydraulic resistances of the lines change; on the other hand, there is also the danger of quality losses due to the sudden freeing of such scale and/or slime deposits. At present, if scale or slime deposits of this sort occur, the attempt is made either to clean the scale and/or slime deposits during operation with the aid of expensive chemical additives or additives that impair the manufacturing process or to carry out a mechanical cleaning when the machine is shut down.